Temporomandibular disorders (TMD) are a heterogeneous collection of disorders characterized by orofacial pain and/or masticatory dysfunction. Parafunctional behaviors, especially clenching and grinding, are presumed to be important initiating and perpetuating factors in TMD (Glaros & Glass, 1993). Recent studies have showed that low-level parafunctional activity in otherwise normal individuals increases pain and can produce the same symptoms of TMD reported by clinic patients. However, there is very little direct evidence that TMD patients engage in more parafunctional activities than others or that the parafunctional activities that they may engage in are related to pain. The specific aims of this project are therefore to test the hypotheses that (1) TMD patients engage in higher rates of parafunctional activities than a comparable non-TMD sample, and (2) parafunctional activities produce and/or increase pain. Both in vivo and laboratory-based studies will be conducted to test the hypotheses. One of the proposed studies will utilize a methodologically sophisticated, ecologically valid self-report technique known as ecological momentary assessment (EMA). EMA data are a sample of the participant's behavior, emotional state, or physiological states in the real world, and these data can be examined for co-variation among the variables in time. The long-term objectives of this research are: (1) to better describe and characterize some basic, essential differences among TMD and non-TMD subjects; (2) to develop an etiological theory of TMD which adequately accounts for the observed differences; and (3) to develop effective treatment programs for TMD derived from our improved understanding of this disorder.